## app/models/posted_item.rb
class PostedItem < ActiveRecord::Base
  # ...
  has_many :item_classifications
  has_many :taxonomies, :through => 'item_classifications'
  # ...
end

## app/models/taxonomy.rb
class Taxonomy < ActiveRecord::Base
  has_many :item_classifications
  has_many :posted_items, :through => 'item_classifications'
end

## app/models/item_classification.rb
class ItemClassification < ActiveRecord::Base
  belongs_to :taxonomy, :counter_cache => :item_count
  belongs_to :posted_item
end

## raises: [plain_text]
Internal Server Error 500
Could not find the association "item_classifications" in model Taxonomy